Fix app engine nox sessions

Author: Jon Wayne Parrott

nox.py

@@ -171,7 +171,7 @@ def _setup_appengine_sdk(session):
         NON_GAE_STANDARD_SAMPLES, CHANGED_FILES)
 
 
-def _session_tests(session, sample):
+def _session_tests(session, sample, post_install):
     """Runs py.test for a particular sample."""
     session.install('-r', 'testing/requirements.txt')
 
@@ -180,6 +180,9 @@ def _session_tests(session, sample):
     if os.path.exists(os.path.join(sample, 'requirements.txt')):
         session.install('-r', 'requirements.txt')
 
+    if post_install:
+        post_install(session)
+
     session.run(
         'pytest',
         *(PYTEST_COMMON_ARGS + session.posargs),
@@ -193,14 +196,13 @@ def _session_tests(session, sample):
 def session_gae(session, sample):
     """Runs py.test for an App Engine standard sample."""
     session.interpreter = 'python2.7'
-    _setup_appengine_sdk(session)
 
     # Create a lib directory if needed, otherwise the App Engine vendor library
     # will complain.
     if not os.path.isdir(os.path.join(sample, 'lib')):
         os.mkdir(os.path.join(sample, 'lib'))
 
-    _session_tests(session, sample)
+    _session_tests(session, sample, _setup_appengine_sdk)
 
 
 @nox.parametrize('sample', NON_GAE_STANDARD_SAMPLES)